The present invention relates to a method for detecting the working condition of batteries which is applicable to non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries. More particularly, the present invention relates to a method for determining a degree of degradation and a remaining capacity of battery.
Portable electronic appliances like notebook-sized personal computers and cellular phones which run on high-capacity secondary batteries are fast growing in recent years. The appliance is usually provided with a remaining battery capacity meter which indicates a remaining serviceable time of the battery to the operator.
A performance of a secondary battery which drives the electronic appliance falls inevitably as a charge/discharge cycle is repeated. But such degradation in performance of battery is indicated to the operator on very few of those electronic appliances. The operator somehow notes a deterioration in performance of battery only from a feeling that the serviceable time of the equipment is getting short.
There have been disclosed a number of methods of detecting the degree of deterioration in performance of secondary battery, which can be classified roughly into:
(1) measuring an internal impedance of a battery, PA1 (2) measuring internal impedances of a battery at plural alternating currents with different frequencies, and substituting these values into a predetermined arithmetic equation, PA1 (3) measuring an electric resistance of an active material of a battery that is a constituent element of the battery, PA1 (4) detecting a voltage of a battery while charging or discharging it at a constant current, and comparing the obtained value with the predetermined reference value, and PA1 (5) counting the number of charge/discharge cycles. PA1 where C.sub.a is a discharge capacity, and K.sub.1 and k.sub.2 are predetermined characteristic values of the battery. On the basis of thus obtained discharge capacity C.sub.a, it is possible to make a consistent and quantitative determination of the degree of degradation of the battery.
Many methods for detecting the degradation in performance of secondary battery have been proposed as mentioned above. But the progress of the degradation in performance of secondary battery is much different depending on its application condition, that is the charging and discharging currents, charging and discharging voltages, and charging and discharging times. There is much difference in degree of degradation between a battery which has subjected to a repetition of a shallow charge/discharge cycle and another which has been subjected to a repetition of a deep charge/discharge cycle even if the numbers of both cycles are same. Therefore, it is difficult to digitize the degree of degradation by any of those methods including the one that simply counts the number of charge/discharge cycles.
On the other hand, some methods for detecting the remaining capacity of secondary battery are now adopted on portable equipments. Those methods are essentially classified into the following two methods:
One is a direct method in which a determination is made merely from a measurement of battery voltage. This is widely used in cellular phones. The detection apparatus for this direct method is relatively low in manufacturing cost but the method involves a shortcoming of low detection accuracy. When an electronic appliance is in operation, the battery voltage always fluctuates depending on an operating mode of the appliance. Therefore, measurement errors will be large if the remaining capacity of battery is determined simply based on the battery voltage with the appliance in operation regardless of any consideration to the operating mode. In the portable electronic appliance such as a cellular phone, the remaining capacity display by this method employs a three- or four-stage lighting system using LED with one end being fully charged condition and the other being empty condition.
The other is an indirect method in which an information of an integrated charging current value is stored on a memory and discharging current value is deducted from the integrated charging current value as the battery discharges. This system is widely employed on notebook-sized personal computers. The indirect method is high in accuracy and can show the remaining capacity in minutes. However, this method needs the memory to store the information of the integrated current value, which pushes up the costs of appliance.
Further, other ideas are also proposed. Among them are methods for detecting the remaining capacity by measuring a voltage drop of battery at pulse-discharging, by utilizing recovery characteristics of battery voltage after pulse-discharging, by measuring a capacitance of battery, or by measuring an impedance at a specific frequency then calculating the remaining capacity from the ratio of the real number component to the imaginary number component of the impedance or a calculation using the imaginary number component and the frequency.